In a production process of a semiconductor element, an original pattern in which a circuit pattern is formed, that is, a mask or a reticle (hereinafter collectively referred to as a mask) is exposed and transferred onto a wafer by a reduction projection exposure apparatus called a stepper or a scanner. Since production of a Large Scale Integration (LSI) requires a large manufacturing cost, it is crucial to improve the production yield. A defect of a mask pattern can be cited as a large factor of degradation in the production yield of a semiconductor element.
Further, in the inspection process, not only is the detection of a defect performed, but also, measurement of a line width (CD) of a pattern formed in a mask is performed, to generate a map of a distribution of a difference value (line width error: ΔCD) between the measured value of the line width and the design value of the pattern. The obtained ΔCD map is then fedback to the mask production process to be used for revising the condition of the process.
Document 1 (Japanese Unexamined Patent Application Publication No: 2012-220388) discloses an inspection apparatus for inspecting a mask by dividing light emitted from a light source to two light paths, wherein one light path illuminates a mask to be inspected by light transmitted through the mask, and another path illuminates the mask by light reflected by the mask, thereby obtaining an optical image of the pattern of the mask by a sensor in which the light transmitted through the mask is incident, and another sensor in which the light reflected by the mask is incident. In this inspection apparatus, a polarized beam splitter is disposed along the light path from the mask to the sensor. Further, this polarized beam splitter is also disposed along the light path for illuminating the mask with the light to be reflected. That is, the light reflected by the polarized beam splitter illuminates, and is then reflected by, the mask, and is incident to the sensor after returning through the polarized beam splitter.
Generally, a mask consists of a quartz glass substrate, and a pattern consisting of a light shielding film comprising a chrome film formed on the surface of quartz glass substrate. The quartz glass substrate has birefringence, and the direction of the birefringence is different depending on the position of the inside of the substrate as a result of distortion, etc. Therefore, the polarized state of the light before transmission through the mask is then changed to a different polarized state after the light is transmitted through the mask, as a result there is a problem in that the light quantity of the light transmitted through the polarized beam splitter is decreased. Specifically, the light before transmission through the mask is circularly polarized light, whereas on the other hand, the light after transmission through the mask is elliptical-polarized light, depending on the difference of the direction of the birefringence in the mask. The light transmitted through the mask is changed by a quarter-wavelength plate from the circularly polarized light to the linearly polarized light having only a p-polarized component for the polarized beam splitter, thereby allowing transmission through the polarized beam splitter. However, because the elliptical polarized light is incident to the quarter-wavelength plate, the light to be incident to the polarized beam splitter is: non-linearly polarized light, or a linearly polarized light having the s-polarized component. As a result the light quantity of the light to be transmitted through the polarized beam splitter is decreased.
In order to resolve the above-mentioned problem, in Document 1, the quartz glass substrate in which a pattern is not formed, is illuminated before the inspection for detecting a defect, and then the transmitted light is incident to the sensor in order to generate brightness distribution data. Then, a brightness value of an actual inspection is corrected using the brightness distribution data. However, it takes a great deal of time to obtain the brightness distribution data for every before the inspection. Document 1 also does not disclose a generation of a ΔCD map.
An object of the present invention is to provide an inspection apparatus and an inspection method that can perform inspection to detect a defect easily and accurately by decreasing an effect by birefringence of a quartz glass substrate, and can obtain an accurate line width error in order to generate an accurate ΔCD map. Other advantages and challenges of the present invention are apparent from the following description.